1. Field of the Invention
The present invention relates to a process for preparation of 3,4-dihydroxybutanoic acid and salts thereof from a D-hexose source, particularly a glucose source containing glucose as a substituent. In particular, the present invention relates to a process for preparing (S)-3,4-dihydroxybutanoic acid and salts thereof.
2. Prior Art
During the course of the development by syntheses for naturally-occurring (R)-3-hydroxy long chain fatty acids, various synthetic routes to (S)-4-bromo-3-hydroxybutanoic acid methyl or ethyl esters were examined. The general approach was to carve out this chiral fragment from a suitably modified carbohydrate structure. Initial attempts involved selective protection and structural modification of methyl alpha-D-glucopyranoside followed by cleavage to yield a 4-carbon fragment containing the required functionalities. Although this approach proved to be quite viable, it proved not to be as direct as we had envisioned.
A reaction in which some of the desired product is generated in a fewer steps from inexpensive starting materials was considered. The treatment of cellobiose, a beta-1,4-linked glucose disaccharide, maltose (the alpha-1,4-linked isomer) and other related compounds with alkali has been shown to generate low yields of the desired material along with D,L-2,4-dihydroxybutanoic acid, glycolic acid, isosaccharinic acids, ketones, diketones, glyceric acids and a myriad of other degradation and condensation products (Corbett, W. M., et al., J. Chem. Soc., 1431-1435 (1955); Green, J. W., J. Amer. Chem. Soc. 78:1894-1897 (1956); and Rowell, R. M., et al., Carbohydr. Res., 11:17-25 (1969)). Starch and cellulose also yield similar compounds in what is known as the "peeling reaction". This process is, generally, thought to have no synthetic potential. Most of the products formed in these reactions are formed from the intermediate dicarbonyl (diulose) compound F shown in FIG. 1 according to a mechanism proposed by Isbell (Isbell, H. S., J. Res. Natl. Bur. Stand., 29:227 (1942)). The dicarbonyl compound F is rapidly attacked by alkali to yield a tarry mixture and the formation of 3,4-dihydroxybutanoic acid (1) and glycolic acid (4) as shown in FIG. 1 in low yields and is slow and oxygen-dependent.
Alkaline hydrogen peroxide rapidly cleaves diketones to give carboxylic acids and treatment of diuloses and other carbohydrates with hydrogen peroxide in this manner has been described (Moody, G. J., Advances in Carbohydr. Chem., 19:149-180 (1964)). The reference does not describe the use of hydrogen peroxide to cleave a glucose source containing a 1,4-glucose linkage. Earlier work on the oxidation of maltose (Glattfeld, J. W. E., et al., J. Amer. Chem. Soc. 40:973 (1918) using base and hydrogen peroxide yielded no 3,4-dihydroxybutanoic acid but gave glycollic acid, arabonic acid, D-erythronic acid, oxalic acid and formic acid. In this work, the reaction was conducted for a very prolonged period (13 days) at room temperature followed by an undefined period at 50.degree. C. The molar proportions of base and hydrogen peroxide were both 8 to 9 fold of the sugar proportion. These conditions cause complete conversion of product to formic acid.
3,4-Dihydroxybutanoic is a valuable chiral building block and the general strategies for obtaining it and its derivatives hinge upon the development of enzymatic systems utilizing beta-ketoesters as substrates (Nakamura, N., et al., Tetrahedron Letters, 30:2245-2246 (1989); Zhou, B., et al., J. Amer. Chem. Soc., 105:5925-5926 (1983); and Nakamura, N., et al., Tetrahedron Letters, 31:267-270 (1990)).